For many applications, such as formation of solar cells using ion implantation, the ability to implant at high current in an efficient manner is needed to reduce production costs. Large area sources may have various configurations.
Known beamline implanters may include an ion source, extraction electrodes, a mass analyzer magnet, corrector magnets, and deceleration stages, among other components. The beamline architecture provides a mass analyzed beam such that ions of a desired species are conducted to the substrate (workpiece). However, one disadvantage of the beamline implanter architecture is that the implantation current and therefore the throughput may be insufficient for economical production in applications such as implantation of solar cells.
Plasma doping tools (PLAD) may provide a more compact design that is capable of producing higher beam currents at a substrate. In a PLAD tool, a substrate may be immersed in a plasma and provided with a bias with respect to the substrate to define the ion implantation energy. This provides the potential for higher beam current, which reduces the time needed to perform an implant. However, one drawback of PLAD systems stems from the need to introduce and purge gas into the system. Initially, the gas pressure inside the process chamber may be in the milliTorr (mTorr) range, while after processing, the process chamber pressure may be required to be less than 10−4 Torr before unloading the substrates. This adds significantly to the cycle time for processing a substrate even when the actual duration of ion implantation is relatively short.
Many present-day applications require not only high throughput but low ion energy. In order to achieve this, typical PLAD systems employ a plasma source, which may be an RF or DC source, and a separate power supply used to independently bias the substrates to be implanted with respect to the plasma. The bias power may be supplied as pulsed DC bias, as an RF bias, etc., in known systems. Although less complex apparatus have been developed for high throughput ion implantation, such as glow discharge DC PLAD (GD PLAD) tools, the latter are not ideally suited for providing low energy, high density plasmas. In the GD PLAD method, a single pulsed-DC wafer bias is applied to a process chamber for both plasma generation and for biasing the substrate to cause ion implantation. However, the low bias-voltage typically cannot generate high-density, uniform plasmas over the substrate region for high-throughput.
FIGS. 1a and 1b show a comparison of known GD PLAD and RF PLAD systems. In GD PLAD system 100, an anode 104 is grounded and a negative voltage DC pulse 114 is applied to a workpiece (substrate) holder 106 and generates a plasma 110, while at the same time supplying a bias to substrate 108 with respect to plasma 110 in order to attract ions 112 for implantation. The implant uniformity can be optimized by adjusting the operating pressure and the physical gap between the wafer and anode. This approach works relatively well at high voltages suitable for high energy ion implantation, but does not work well at low energies, e.g. <3 keV), because of the low plasma density generated at such low voltages, and does not work at all below the Paschen curve.
The RF PLAD system 120, in contrast, employs an external plasma source 124 to independently generate a plasma 130. In this case, the substrate holder 126 and substrate 128 are biased at negative potential using a separate voltage supply (not shown) that supplies a pulsed DC bias 134 or RF bias 136, so as to attract ions 132 for implantation into substrate 128. Because the plasma 130 is generated by plasma source 124, the plasma density can be maintained at a high level even if a low bias voltage is supplied to substrate holder 126, since the ionization of plasma species is not primarily dependent on the voltage applied to the substrate holder, as is the case in GD PLAD system 100. While the RF PLAD system affords the ability to generate high density plasmas at low ion energy, system complexity and expense are undesirable. In addition, known GD PLAD and RF PLAD systems such as systems 100 and 120 do not provide a means for patterned implantation to be performed on workpieces. In view of the aforementioned drawbacks, it will therefore be apparent that a need exist to improve PLAD type ion implantation systems, especially in the case of high throughput low energy ion beams.